1. Field of the Invention
The present invention relates to a projection exposure apparatus and a device manufacturing method both of which are suitable for use in the lithography process of exposing a wafer surface to an electronic circuit pattern formed on the surface of a mask or reticle (hereinafter referred to as a "reticle") by projection exposure or scanning exposure via a projection optical system, thereby providing a device of a high integration level, during the manufacture of semiconductor devices such as ICs or LSIs, image pickup devices such as CCDs, display devices such as liquid crystal panels, or devices such as magnetic heads.
More particularly, the present invention relates to a projection exposure apparatus and a device manufacturing method both of which are suitable for use in a case in which a reticle and a wafer are aligned with high accuracy and the electronic circuit pattern on the surface of the reticle is exposure-projected on the surface of the silicon wafer at a high integration level.
2. Description of the Related Art
Conventionally, when semiconductor devices, liquid crystal panels or the like are to be manufactured with photolithography techniques, a projection exposure apparatus (stepper) is used which transfers patterns from a reticle surface to a photosensitive substrate such as a wafer or a glass plate coated with a photoresist or the like, by exposure via a projection optical system.
The recent semiconductor technology, in particular, has been proceeding into a far larger scaling stage, and the resolution of, for example, from a linewidth of 0.25 .mu.m to a far finer pattern, has been becoming a matter of argument. Techniques which play the central role in this situation are optical exposure techniques represented by steppers. The performance of a projection lens (projection optical system) which is a performance index of the optical exposure techniques is being improved in three major directions, i.e., shorter wavelength, larger field size and higher NA (numerical aperture). In view of the shorter wavelength, lithography using light from an ArF excimer laser is vigorously being developed as the next-generation techniques.
One important item of the optical exposure techniques is to maintain the same optical performance at all times. In lithography using an ArF excimer laser, it is known that a usable glass material has the characteristic of absorption of light and the optical performance of a projection optical system varies due to the absorption of light. The absorption of light has also already been known in the field of lithography using light such as i-line. In a stepper using an ArF excimer laser, since its depth of focus becomes smaller, it is necessary to control its optical performance with higher accuracy than with conventional accuracy. The optical performance to be controlled includes various aberrations of the projection optical system. Among these aberrations, longitudinal astigmatism, curvature of field and the third-order distortion as well as higher-order distortion are most difficult to correct. Furthermore, how to correct the rotationally asymmetrical magnification of a target itself becomes a new problem.
Longitudinal astigmatism will be described below with reference to an example. A scan type of a projection optical system called a scanner has recently vigorously been developed to enlarge the exposure area of a wafer surface. In the scan type of a projection optical system, its illumination area has a slit-like shape and a reticle and a wafer are synchronously scanned with respect to the slit-like shape. If this slit-like shape of exposure light is used, since the shape of the slit is not rotationally symmetrical, heat distribution asymmetrical about the optical axis of the projection optical system occurs due to the absorption of light into glass material, and longitudinal astigmatism (longitudinal astigmatism) occurs in the projection optical system.
However, projection optical systems of the type used in conventional projection exposure apparatus are not provided with means for correcting various asymmetrical aberrations due to the absorption of exposure light by varying the optical performance of the projection optical system, so that it has been impossible to effectively correct asymmetrical longitudinal astigmatism due to the influence of the absorption of exposure light. For this reason, passive measures have conventionally merely been taken, such as a method of limiting the amount of exposure light to be made incident on a projection system and restraining the occurrence of various aberrations.
Curvature of field will be described below. Since curvature of field relates to the Petzval sum, a method of varying the power (refractive power) of an optical element such as a lens or a mirror is known as a method of correcting a variation in curvature of field due to the absorption of exposure light into a projection optical system. However, conventionally, no optical elements having variable refractive power have been applied to the projection optical systems of projection exposure apparatus, and it has been impossible to effectively correct the influence of absorption of exposure light. For this reason, passive measures have conventionally merely been taken, such as a method of limiting the amount of exposure light to be made incident on a projection system and restraining the occurrence of various aberrations.
Rotationally asymmetrical magnification will be described below. In addition to an improvement in resolution, the alignment accuracy of patterns which are stacked in layers is a factor required for optical exposure techniques for manufacturing semiconductor devices.
A technique called global alignment is generally widely used as an alignment method. The error of global alignment is mainly divided into an intershot component which is the error between individual printed shots and an intrashot component which is the error inside each individual shot. Recently as the field size has become larger, how to reduce the error of the intrashot component has become a greater problem. In an actual wafer, asymmetrical distortion occurs during the process of the wafer.
For example, if an error of 2 ppm in magnification is present as an asymmetrical uncorrectable component for a field size of 22 mm, the following error occurs: EQU 22 mm.times.2 ppm=44 nm,
and the error has a value close to 1/5 times the resolution performance of a linewidth of 0.25 .mu.m. As viewed from an overlay budget, it is apparent that this value is not within allowable limits, and how to reduce the intrashot component has become a great problem to the projection exposure apparatus.
With regard to distortion, known means is available as a method of controlling magnification and the third-order distortion. For example, there are a method of moving a plurality of optical elements with respect to each other in a projection optical system along the optical axis thereof, and a method of controlling the pressure in the gas sealed between optical elements. Magnification is a basic quantity of an optical system and can be varied without causing variations in other aberrations, but as to the third-order distortion, an optical system needs to be designed from the beginning in view of the correction of the third-order distortion because the correction of the third-order distortion offers problems such as variations in other aberrations and a narrow range of adjustment. In particular, if various image-forming methods which are generically called image enhancement such as off-axis illumination techniques or phase-shifting masks are adopted, the matching of distortion between image-forming methods becomes a problem. In this case, it is a great problem to establish the techniques of arbitrarily controlling the third-order distortion without imposing a load on design.
Higher-order distortion will be described below. In a scan type of projection exposure apparatus, it is possible to correct through scanning an asymmetrical magnification difference, for example, the difference in magnification between the x- and y-axes taken in a direction perpendicular to the optical axis of the apparatus. Regarding the direction of scanning, the averaging of distortion is effected by an averaging effect which accompanies scanning. For example, if a slit is longer in the x-axis direction and scanning is performed in the y-axis direction, the scanning results in the fact that magnification in the y direction is adjusted by controlling the synchronism of the scanning, and optical distortion such as so-called third-order and fifth-order distortion is reduced to a small value by averaging within the slit.
However, since no averaging is effected in the x direction, it is necessary to optically control distortion with high accuracy. A greatest problem encountered when magnification and the third-order distortion are corrected by known techniques is higher-order distortion of the fifth order or higher. It is said that to correct this higher-order distortion is a great problem to be solved.
Japanese Laid-Open Patent Application No. Hei 7-183190 has proposed a projection exposure apparatus which is capable of adjusting optical characteristics rotationally asymmetrical about the optical axis of its projection optical system which remain in the projection optical system for projecting a pattern from a mask surface onto a wafer surface.
According to Japanese Laid-Open Patent Application No. Hei 7-183190, in the projection exposure apparatus having an illumination optical system for illuminating a first object and a projection optical system for projecting an image of the first object illuminated by the illumination optical system onto a second object at a predetermined reduction magnification, optical means having power rotationally asymmetrical about the optical axis of the projection optical system is disposed between the first object and the second object, and the optical means is disposed for rotation about the optical axis of the projection optical system or for movement along the optical axis of the projection optical system in order to correct optical characteristics rotationally asymmetrical about the optical axis of the projection optical system which remain in the projection optical system.
Various proposals, such as the simplest method of effecting focus correction during exposure, have heretofore been made with respect to a method of correcting a variation in optical performance due to the absorption of exposure light into a projection optical system. However, in the age of the scan type of projection exposure apparatus (scanner), it has newly been found out that aberration rotationally asymmetrical about the optical axis occurs, and this finding has been becoming a great problem. This is a phenomenon which occurs because illumination light is formed in a slit-like shape asymmetrical about the optical axis, i.e., the intensity distribution of light differs between the longitudinal direction and the lateral direction of the slit-like shape, and longitudinal astigmatism which causes the difference in focus between longitudinal lines and lateral lines to vary on the optical axis may occur in an extreme case. The fact that no longitudinal astigmatism occurs so long as no large exposure load is applied to the projection optical system, but if a large exposure load is applied to the projection optical system, longitudinal astigmatism occurs signifies the unstableness of the entire system and is a great problem to the scanner.
Curvature of field is one kind of optical performance which varies when a projection optical system absorbs light in a normal stepper or a scan type of projection exposure apparatus. As described above, there is a method of correcting curvature of field by varying the power of an optical element. However, there is the problem that it is, in general, very difficult to vary the power of an optical element with high accuracy and satisfactorily correct curvature of field.
In addition, to achieve higher resolution, it is necessary to align a reticle and a wafer with higher accuracy. However, if a rotationally asymmetrical magnification difference occurs in the wafer which is a target object, alignment accuracy lowers.
To improve alignment accuracy, the control of magnification becomes a problem, but it is customary that projection optical systems of the types used in steppers have only the function of correcting magnification rotationally symmetrical about their optical axes. However, in an actual semiconductor process, if x, y coordinates are set over a wafer surface according to the directionality of a pattern to be printed, there is a case in which the magnification of extension for each process differs between the x direction and the y direction, i.e., between the longitudinal direction and the lateral direction of the wafer surface, thereby imposing limitations on an improvement in the overall alignment accuracy. Accordingly, for example, if there is a difference of 2 ppm in extension between the x direction and y direction, there is the problem that an uncorrected difference remains as described above.
In the scan type of projection exposure apparatus, although the value of distortion is improved in the form of averaging with respect to the scanning direction, there is the disadvantage that the merit of averaging is not obtained with respect to a direction perpendicular to the scanning direction. As to distortion in particular, there are known methods of controlling magnification and the third-order distortion.
For example, there are a method of relatively moving a plurality of optical elements in a projection optical system along the optical axis thereof and a method of varying the pressure of the gas sealed between optical elements of a projection optical system. However, higher-order distortion of the fifth order or higher has been difficult to correct with any of these known methods. Conversely, there is even a case in which the third-order distortion is difficult to correct.
In particular, if various image-forming methods which are generically called image enhancement such as off-axis illumination techniques or phase-shifting masks are adopted, the matching of distortion between image-forming methods becomes a problem. According to our analysis, it has been found out that most problematic elements in this case are the difference in magnification between the x direction and the y direction and higher-order distortion if magnification and the third-order distortion are corrected by known means. Since the difference in magnification between the x direction and the y direction can easily be corrected in the scan type of projection exposure apparatus, how to correct and control higher-order distortion becomes a problem.
The projection exposure apparatus proposed in Japanese Laid-Open Patent Application No. Hei 7-183190 has the problem that if the optical means having the rotationally asymmetrical power are driven, a plurality of aberrations vary, so that only a desired aberration is difficult to correct.